Control circuit for sound reproduction



Nov. 30, 1943. R. A. BIERWIRTH l CONTROL CIRCUIT FOR SOUND REPRODUCTIONA 3 sheets-sheet 1 Filed June 1, 1942 WWNRwDM. MQW,

.INVENTOR Zerw A TTORNE Y Nov. 3o, 1943.

R. A. BIERWIRTH CONTROL CIRCUIT FOR SOUNDREPRCDUCTION Filed June l, 1942 '3 Sheets-Sheet 2 I .I .Y

lmentor .www

Gftorneg NOV. 30, 1943. R A. B|ERW|RTH l 2,335,575

coNTRoL CIRCUIT FOR souNn REPRODUCTION I Filed June 1, 1942 3. sheets-Sheet 5 om o o Ammo IQ Gttorneg Patented Nov. 30, 1943 CONTROL CIRUIT FOR SOUND REPRGDUCTION Rudolph A. Bierwirth, Haddon Heights, N. signor to Radio Corporation ot America,

poration of Delaware Application June 1, 1942, Serial No. 445,287

14 Claims.

This invention relates to electrical current control systems and particularly to a sound reproducing system wherein the reproduction of the sound is under control of a control record.

Sound reproducing systems for theatres wherein the volume of the reproduced sound is either under control of its own, amplitude variations or under` control of a separate control record are well known in the art.l The use of multiple speakers for public address and theatre sound reproduction are also well known. The present invention is directed to a control track type of system using. multiple speakers wherein one or more center speakers generally located behind the picture screen are flanked by one or more side speakers across the proscenium of the theatre.

In the operation of such a system it has been found desirable for certain types of sounds being reproduced, to vary the volume of reproduction independently of the normal volume variations and to distribute or vary the volume of the sound being reproduced by a center speaker or speakers with respect to the side speakers and vice versa. To obtain the variations in volume, variable gain amplifiers are employed, the variation of the grid bias on the ampliier tubes varying the gain thereof and consequently the volume of the reproduced sound. In embodiments of the system wherein the volume of the sound is distributed differently between the center and side speakers, a variable gain amplifier for each group of speakers is employed, together with multiple rectifier control units for controlling each variable gain ampliiier for the respective speakers in a different manner as will be described hereinafter. It has been found that by the use of electronic devices for controlling the distribution, of the sound between the speakers as well as for varying the transmission of the signal currents, that an accurate and smooth variation may be obtained. Although only two particular arrangement of signal 'and control circuits are being described below to illustrate the invention, it will be realized that other combinations and variations are obtainable by varying the adjustment and arrangement of the control elements.

The principal object of the invention, therefore, is to facilitate and improve the reproduction of sound. u

Another object of the invention is to control the amplitude of reproduction of sound from al sound record in accordance with a control record and to vary the volume distribution of the sound among a plurality of loudspeakers.

yA further object of the invention is to provide J., asa cora sound reproducing system in which the reproduced sound is under control of a control record for varying the distribution of the volume of the sound among a plurality of loudspeakers.

A further object of the invention is to provide a sound reproducing system from a sound record in which the volume and the distribution of the sound is under control of a separate control record.

A further object of the invention is to provide a system for varying the amplitude of reproduced sound in alinear relationship with respect to the amplitude variations of a control current.

Although the novel features which are believed to be characteristic of this invention are pointed out with particularity in the claims appendedl hereto, the manner of its organization and the mode of its operation will be better understood by referring to the following description read in conjunction 'with the accompanying drawings forming a part hereof, in which:

Figure 1 is a block diagram of one embodiment of the invention;

Figure 2 is a block diagram of a second embodiment of the invention; V Figure 3 is a schematic circuit diagram of the variable gain ampliiiers and rectifier control units of the embodiment shown in Fig. 1;

Figure 4 is a graph showing the operation of the circuit of Fig. 3; and

Figure 5 is a schematic circuit diagram of the variable gain amplifier and the rectiiier control unit of Fig. 2.

Referring now to Fig. 1, a sound record and control record, such as shown in Burrill Patent t No. 2,270,610 of January 20, 1942, or a control record of any other type, are translated to electrical currents by a signal photoelectric cell i and a control track photoelectric cell 6. The signal currents are ampliiied in amplifier 8, then passed to variable gain ampliiier 9 which feeds the variable gain amplifier l0 and a power amplifier l2 in parallel, the output of power amplifier l2 being connected to a loudspeaker I3 which may be the center speaker or group of speakers behind avmotion picture screen. The output of the variable gain ampliiier i0 is fed to a power ampliiier l5 and is then impressed upon loudspeaker units I6 and i1 which may be the speakers on either side of the center speaker I3.

The output of the control track photoelectric cell 6 is fed to an amplifier 20, then to a rectifier control unit 2|, the output of this latter unit controlling both variable gainampliiiers 9 and I0. From the diagram shown in .'liig.` 1 it is to be observed that the signal currents are impressed upon two variable gain amplifiers S-and I0, one of which feeds the center speaker and.

the other the side speakers. The control cur- `rents from the photoelectric cell 6 individually control each variable gain amplifier. The manyner in which this control is effected will now be Vto amplifier 8 and an output transformer 26 p which may be connected to amplifier I2, the amplifier 8 including a pair 0f exponential pushpull tubes 21 and 28 which may be the type known as RCA 6K7s. The variable gain amplifier I of Fig. 1 is shown in the lower right-hand corner of the circuit drawing enclosed within the dotted lines 30, this unit comprising two single amplifying stages employing tubes 3| and 32 connected in series by a resistance-capacity couling. p The control currents from the amplifier 20, Fig. 1, are fed over a'transformer 34 to a rectifier 35, the output of which is fed to a second rectifier 36. .Plate and grid potentials and heating currents for the units just described are obtained from a power supply unit comprising a transformer 38, a rectifier 39 and a filter 48. A voltage regulator tube is shown at 4|. One preferred adjustment of the above units is such that with no control signal the grid bias applied to the grids .of tubes 21 and 28 over resistance 55 of the voltage divider 42-41-55-43 and 44 will reduce the gain of tubes 21 and 28 about 10 db below normal, while thegrid bias on tubes 3| and 32 will reduce their gain to minus infinity or, in other words, to a point where no signal currents can pass therethrough.

Referring now to the rectifier 35, a small direct current bias from the rectifier 39 is provided for rectifier 35 by the difference in setting of the sliders on resistances 43 and 44 overconductors 45 and 46 and resistance 48, the rectifier output of the rectifier 35 occurring across resistance 48. The adjustment of vthis rectifier bias: is to prevent any residual control current flow from appearing in the output of the rectifier 35. In this arrangement, a bias circuit for the tubes 21 and 28 is over conductor 5l),A conductor 5I, resistance 52, conductor 53, conductor 54,

. resistance 55 and conductor 56. Therefore, current flowing in resistance 52 from the left-hand cathode of the rectifier 36 will increase the bias on the tubes 21 and 28 in a positive direction and correspondingly increase their gain. Current, however, will not flow through the resistance 52 until the bias placed on rectifier 36 by resistance 44 is overcome by direct current from rectifier 35, so that the gain of the variable gain amplifier 21-28 will not change the amplification of the signal currents passing therethrough until a certain amplitude of control current is reached. Thus, the setting of the slider on resistance 44 determines the amplitude of control current from which the amplitude of the signal currents is varied from normal.

Contrary to the above, the gain of tubes 3| and 32 will be varied bythe smallest amplitudes of control current over a circuit from the cathode of rectifier 35 over a portion of resistance 48, conductor 59. resistance 60, conductor 6I and resistances 62 .and 63- in parallel to the grids of the tubes 3| and 32. Thus, the control currents will immediately vary the bias on tubes 3| and 32 in a positive direction, increasing their vgain and the amplitude of the signal currents to the side speakers I6 and I1 of Fig. 1. During this variation, however, and as long as the control currents are below an amplitude which will overcome the bias on rectifier 36, the amplitude of the signalcurrents to the center speaker I3 will not be varied except, of course, in accordance with their normal variations on the signal track.

Continuing the operation of the circuit, the gain of tubes 3| and 32 will continually increase with no change in gain in-tubes 21 and 28 as the amplitude of the control current increases. At a. predetermined control current amplitude where the bias on rectifier 36 obtained from resistance 44 is overcome, current will flow through both sides of rectifier 36. The flow of current through the left-hand side of the rectifier 36 and through resistance 52 will shift the bias on tubes 21 and 28 in a positive direction, thus increasing their gain in proportion to the amplitude of the control currents.` As the voltage on the right-hand side of rectifier 36 approaches zero and current starts to iiow, the internal resistance of the rectifier will become low compared to the resistance' 60, so that'further increases in control current will not appreciably change the bias applied to the grids of tubes 3| and 32 and the gain will remain constant. Since current will begin to flow through both sides of rectifier 36 at the same instant since both sides have the same bias from resistance 44, variable gain amplifier 9 will begin increasing its gain when the gain of variable gain amplifier I0 becomes constant. Thus, setting of the bias on rectifier 36 determines the point at which the signal currents to the side Aspeakers cease to be varied in amplitude with respect to the vcenter speaker and the point at which the amplitude of the signal currents to all the speakers begins to vary conjointly.

To further illustrate this operation reference is made to Fig. 4 wherein the uppermost dotted curve represents the gain variation of the variable gain amplifier 21-28 and the uppermost solid curve represents the gain variation of variable gain amplifier 3 I--32, the change-over point being shown by the break in these two curves which occurs at the same amplitude of control current. The solid curve continues downwardly to minus infinity. To show the corresponding bias variation on the two amplifiers reference is made to the lower set of curves wherein the broken line curve shows the variation in bias on amplifier 21-28, while the full-line curve represents the change in bias on tubes 3I-32. It will be observed that the bias variations are linear while the variation in gain of tubes 3I-32, which are not variable mu tubes, is logarithmic.

Referring now to Fig. 2 showing another ernbodiment of the invention, the same signal pickup photoelectric cell 5 and control track pickup photoelectric cell 6 are shown connected to their respective amplifiers 8 and 20, respectively, the output of amplifier 8 now feeding a single variable gain amplifier 9 which is connected to power amplifier I2 which simultaneously feeds all the speakers I3, I6 and I1. The rectifier control unit 2| connected to amplifier 2D now controls only the single variable gain amplifier 9 since al1 speakers are connected thereto. These control circuits are shown in Fig. 5 and will now be described.

The output of amplifier 8 is fed over a T-pad 10 to a transformer 1I and then to variable mu tubes 12 and 13, the output of which is fed over transformer 'l5 to power amplider i2. In order to remove from and insert into the system the variable gain ampliiier 12-3, switches 16m-'il are provided, such a switching system also being applicable to the system of Fig. 3, is desired.

Two amplifying tubes 19 and 8d for the control currents are shown connected by a resistancecapacity coupling, the output of tube 80 being impressed upon a rectifier l8l over transformer 82. After rectiucation of the 'signal control cur rents in rectifier 8| they are fed through a filter 83 to a rectiiier 8d which is similar to rectier 38 of Fig.3, except that the anodes are connected together as well as the cathodes. Plate and grid potentials and heater currents are supplied to the units just mentioned from a power pack including a transformer 85, a rectier 86 with the usual iilter resistances 81 and condensers 88. Voltage regulator tubes are shown at 89.

The grid biasing circuit for the variable gain amplifier tubes 'I2 and 13 is over conductor 9|, the lower tapped portion of resistance 92, resistance S3 and conductor 54, normal bias being supplied by the voltage drop acrcss resistance 93. This arrangement provides no bias for rectifier 8l while a negative bias determined by the` adjustment of slider 9G on resistance el controls the amplitude of direct current from rectiiier 3i necessary to produce a current ow through rectiiier 84. l

The above system operates similarly to the system of Fig.`3 inthat the bias on rectifier dit is adjusted at iid so that at any predetermined level of control current, direct current will flow through rectifier tt and consequently through resistance 92 which will increase the bias on tubes 'l2 and 'I3 in a positive direction, thus increasing their gain, and consequently increasing the amplitude of reproduction of the signal currents in all of the speakers I3, I6 and Il of Fig. 2.

From the above it is to be noted that the first rectifier 8l is unbiased while the rectier 84 has a negative bias placed thereon over resistance 91. The use of two serially-arranged rectiiiers permits a linear relationshiplto be obtained-,between the control signal variations in amplitude and the variations in amplitude of the signal output. It is well known that a biased rectifier has a curved characteristic between A. C. input and D. C. output because of the variation in angle with amplitude, or in other words, the fact that the alternating current is impressed over longer time intervals at the higher amplitudes. However, an unbiased rectiiier with a high load impedance with respect to its tube impedance has substantially a straight line or linear'characteristic. Thus, the first rectifier 8l is unbiased and is provided with a high load impedance; namely, the biased rectifier 84, the latter providing the selective control at any predetermined level, this second rectifier also having a high load impedance with respect to its tube impedance. The second rectifier although biased, does not exhibit a curved characteristic, however, because it has la direct current input and therefore serves similarly to a, simple switch to pass the direct current output of the rst rectiiier BI at a predetermined bias point. Thus, this arrangement of rectifiers provides the desired linear relationship between the amplitude of the A. Crcontrol signal and the direct current output from the second rectifier over a predetermined selective input level range as shown by the straight sec-a tion of the upper broken line curve of Fig. 4.

I claim as my invention:

1. A sound reproducing system comprising means for obtaining electrical currents correspending to e, signal, means for transmitting said currents, and means for obtaining elec trical currents for controlling thetransmission of said signal currents, said last-mentioned means including serially connected rectifiers for linearly varying the amplitude of a predeter mined portion of said signal currents during transmission in accordance with the amplitude of said control electrical currents.

2. A sound reproducing system in accordance with claim 1 in which a plurality of loudspeakers are provided for.- translating said signal currents into sound waves, said control currents varying the volume of reproduction of said signal among said loudspeakers.

3. A sound lreproducing system in accordance with claim 1 in which said transmission means includes a plurality of variable gain amplifiers separately and differentlyl controlled by said control currents'. f

4. A sound reproducing system comprising a source of signal currents, means for variably amplifying said currents, means for translating said currents into sound waves at a plurality of different places, a source of control currents, and means for utilizing said control currents to vary the ampliiication of said signal currents being reproduced at certain points while maintaining the amplification oi said signal currents constant at other points of reproduction.

5. A soundreproducing system in accordance with claim 4 in which said last-mentioned means includes serially connected rectiiiers for said control currents, one of said rectiflers controlling solely the amplification of said signal currents being reproduced at' certain points and another of said rectiflers simultaneously preventing the amplification of said particular signal currents while varying the amplification of said signal currents being reproduced at other points.

6. A sound reproducing system in accordance with claim 4 in which said amplifying means includes a plurality of variable gain ampliiiers and l said last-mentioned means includes a plurality of rectiiiers for said control currents, one of said rectiflers varying the gain of one of said variable gain ampliers over a certain volume range of said control currents, and another of said rectifers preventing the varying of the gain of said one variable gain amplier over'another volume range of said control currents.

7. A sound reproducing system in accordance with claim 4 in which 'said amplifying means includes a plurality of variable gain amplifiers and said last-mentioned means includes a plurality of rectiers for said control currents, the output current of one of said rectiers varying the gain of one of said variable gain amplifiers over' a certain volume range of said control currents while ineiective to vary the gain of said other variable gain ampliiier, the output current of the other of said rectifiers preventing the varying of the gain of said one variable gain amplier over another volume range of said control currents while simultaneously varying the gain' of another of said variable gain amplifiers.

8. A current control system comprising a pair of variable gain amplifiers, a plurality of rectiiiers,

two of said parallel connected rectiers being connected in series toa third rectifier, means for biasing said two parallel connected rectiiers to prevent current ow therethrough until the level of onone' of the output current from said third rectiiier is of a predetermined value. means for varying the bias said variable sain ampliiiers with the current from said third rectiiler, means varying of the bias on said output for preventing the one variable gain amplifier by said third rectifier with the output current from one of said parallel connected rectiiiers, and means for varying the bias on said other variable gain amplifier with the 'output current from said other parallel connected rectifier.

9. A current control system in accordance with claim 8 in which a common source o! signal currents is provided for said variablel gain ampliers.

` l0. A current control system in accordance with claim 8 in which a common source of signal currents is provided for said variable gain ampliilers and separate sound reproducers are connected to the output circuits of each of said variable gain amplifiers.

11. In a sound reproducing system, the combination of a source of signal currents, a variable gain amplifier for variably amplifying said currents, a sound reproducer connected to said variable gain amplier, a source of control currents, a rectiiler for said control currents, a

second rectier connected to the output circuit oi said ilrst mentioned rectifier, the output circuit of said second amplifier being connected to the bias control electrodes of said variable gain ampliiier, andV means for biasing said second rectiiler to prevent through until a predetermined level oi control current to said iirst rectiner has been reached.

12. A sound reproducing system in accordance with claim 11 in which said first rectifier is unbiased and has a high load impedance with respect to its tube impedance.

13. In a sound reproducing system 'the combination of a source of signal currents, variable gain ampliiiers for said currents, a source of control currents, a first rectifier for said control currents, the output current of said rectiner varyins the gain of one of said variable gain amplifiers, a second rectifier for preventing said first rectiiier from varying the gain of said one variable gain amplifier, a third rectifier for varying the gain of said other variable gain ampliiier, and means for biasing said second and third rectiiiers to obtain a predetermined starting point for the operation thereof.

14. A sound reproducing system in accordance with claim 13 in which the value of said bias on said second and third rectiilers determines the amplitude of said control currents at which the gain of said first mentioned variable gain 'amplifier is maintained constant and said second mentioned variable gain ampliiier functions to vary the amplitude of said signal currents being transmitted therethrough.

RUDOLPH, A. BIERWIR'ITI.

the iiow of current there- 

